The growing market for portable wireless communication systems (e.g. wireless phones, wireless local area networks [WLANs], and global positioning systems [GPS]) has increased the need for low-cost and high-performance receivers. Thus, in view of their relatively simple implementation and low cost, very low intermediate frequency (VLIF) circuits are being increasingly used in wireless receivers (instead of other architectures such as superheterodyne and direct conversion).
VLIF receivers require stringent balancing of their IQ paths to maintain acceptable image rejection. Even with careful analogue design, dedicated systems are required for such balancing. However, these systems require calibration to counter the effects of process, temperature, supply and frequency variation.
Calibration can be accomplished online (i.e. whilst receiving an incoming signal) or offline (i.e. using a dedicated training signal). However, recent studies have shown that adaptive algorithms used in online calibration, do not converge rapidly enough to meet the demands of an enhanced GPRS (EGPRS) standard. Similarly, providing dedicated hardware for offline calibration is proving very costly. In general prior art systems for calibrating balancing circuits require three different frequency sources, namely                a real-valued radio-frequency (RF)-tone tunable to any frequency (fRX) in a designated mobile phone frequency band (and thereby generate a test tone for the balancing circuits);        a complex-valued RF-local-oscillator (LO) tunable to a small-frequency-offset (f0) (approximately 100 KHz) from the real-valued RF-tone; fRX and        a digital-local-oscillator (DLO) capable of generating a complex-valued digital intermediate frequency (IF) tone (of frequency f0) and a sampling-frequency (fS) matching that of analogue to digital converters (ADCs) in the balancing circuits.        
In particular, US patent Application US20050008107 describes a receiver which comprises a mechanism for correcting frequency dependent I/Q phase error, wherein the receiver employs a dedicated RF tone generator to drive its mixer circuits. U.S. Pat. No. 6,931,343 describes an on-signal calibration system which uses the I and Q signals of a transmitter to remove distortions in an RF output signal. Similarly, UK Patent GB2406984 describes a method and arrangement for self-tuning I-Q balancing for an I-Q radio receiver.